The enzyme aldose reductase is responsible for the catalytic conversion of aldoses, such as glucose and galactose, to the corresponding alditols, such as sorbitol and galactitol respectively, in warm blooded animals such as man. Alditols penetrate cell membranes poorly and, once formed, tend to be removed only by further metabolism. Consequently, alditols tend to accumulate within cells where they are formed, causing a rise in internal osmotic pressure which may in turn be sufficient to destroy or impair the function of the cells themselves. In addition, raised alditol levels may result in abnormal levels of their metabolites which may themselves impair or damage cellular function. The enzyme aldose reductase has a relatively low substrate affinity and is generally only effective in the presence of relatively large concentrations of aldose. Such large concentrations are present in the clinical conditions of diabetes (excessive glucose) and galactosemia (excessive galactose). Consequently, aldose reductase inhibitors are useful in the reduction or prevention of the development of those peripheral effects of diabetes or galactosemia which may be due in part to the accumulation of sorbitol or galactitol, respectively, in tissues such as the eye, nerve and kidney. Such peripheral effects include, for example, macular oedema, cataract, retinopathy, neuropathy and impaired neural conduction. Although a number of aldose reductase inhibitors have been discovered and clinically evaluated, there is a continuing need for alternative inhibitors. The present invention is based in part on this need and on our discovery of the unexpected inhibition of the enzyme aldose reductase by certain benzheterocyclyl sulphone derivatives.